In the context of an increase of recent communication traffic, demands for optical communication transmission apparatus have been increased. In recent years, not only an optical repeating node which has been introduced to trunk networks but also the optical communication transmission apparatus have been actively introduced to local networks, and further, optical networks have been formed in subscriber systems. Thus, the optical communication system undertakes an important role in world information networks.
As a typical optical communication system, an optical amplification-repeating transmission system is a mainstream, which arranges an optical repeating node provided with a WDM optical amplifier, such as an erbium doped fiber optical amplifier (EDFA) or the like, on a transmission path, to achieve the high reliability at a low cost and also to realize the large-capacity and long-distance transmission.
FIG. 7 is a diagram typically illustrating one example of changes in optical power and OSNR according to a transmission distance, in a typical optical amplification-repeating transmission system. In the system in FIG. 7, a WDM light is transmitted from an optical transmitter (Tx) 101 to a transmission path 102, and the WDM light which has been propagated through the transmission path 102 to be attenuated, is amplified by an EDFA 103 in an optical repeating node before an optical level thereof becomes lower than a required optical level, to be output to the transmission path in a subsequent repeating span. The attenuation of the WDM light in the transmission path 102 and the amplification thereof in the optical repeating node are repetitively performed, so that the WDM light is repeating-transmitted from the optical transmitter 101 up to an optical receiver (Rx) 104.
In such an optical amplification-repeating transmission system, if an inter-node repeating distance becomes longer, a loss in the transmission path 102 is increased. Further, in the case where various types of functional optical components are arranged on an optical signal transmission route, since losses of the functional optical components are added, the repeating loss is further increased. Therefore, an optical input level to the EDFA 103 in each optical repeating node is decreased, so that an OSNR (Signal-to-Noise Ratio) indicating an intensity ratio between a signal light and a noise light is decreased. Incidentally, an OSNR of a signal light output from the EDFA 103 is typically defined by the following formula (1).
                                                                        OSNR                ⁡                                  [                  dB                  ]                                            =                            ⁢                                                signal                  ⁢                                                                                                    ⁢                                                                                                  ⁢                  light                  ⁢                                                                          ⁢                                      level                    ⁡                                          [                      dBm                      ]                                                                      -                                  noise                  ⁢                                                                          ⁢                  light                  ⁢                                                                          ⁢                                      level                    ⁡                                          [                      dBm                      ]                                                                                                                                              =                            ⁢                              optical                ⁢                                                                  ⁢                input                ⁢                                                                  ⁢                                  level                  ⁡                                      [                    dBm                    ]                                                  ⁢                                                                  ⁢                to                ⁢                                                                  ⁢                EDFA                                                                                                                          -                                    ⁢                  NF                                ⁢                                                                  ⁢                                                      (                                          Noise                      ⁢                                                                                          ⁢                      Figure                                        )                                    ⁡                                      [                    dB                    ]                                                  ⁢                                                                  ⁢                of                ⁢                                                                  ⁢                EDFA                            -              constant                                                          (        1        )            
Further, an OSNR of a reception light in the optical receiver 104 (hereafter, the OSNR of the optical signal reaching a reception end is to be referred to as “received OSNR”) is the sum of OSNR in each repeating span. Namely, if the OSNR in each span is “OSNRi” (i=1, 2, . . . , n), “OSNRtotal” of the received OSNR can be calculated by the following formula (2).OSNRtotal=ΣOSNRi  (2)
If a span of long repeating intervals is arranged on the transmission route for the optical signal, a value of the received OSNR is decreased under an influence that a value of the OSNR in this span is decreased. If the received OSNR is decreased, a signal waveform is degraded to increase a reception error possibility.
As one measure for avoiding the decrease of received OSNR due to such an increase of repeating loss, it is effective to apply a distributed Raman amplifier (DRA) to each span or the span of long repeating intervals on the system to thereby increase the optical input level to the WDM optical amplifier such as EDFA or the like, so that the received OSNR in the entire system is increased to thereby improve transmission characteristics.
FIG. 8 illustrates one example of an optical amplification-repeating transmission system in which the distributed Raman amplifier is applied to each repeating span. In the system of FIG. 8, a pumping light source (LD) 105 for Raman amplification is disposed to each span, so that a pumping light output from each pumping light source 105 is supplied to a transmission path 102 from a signal output end side.
As illustrated in FIG. 9 for example, if the pumping light source 105 is OFF (a broken line in the figure), an optical level of signal light propagated through each span is gradually decreased due to a loss in the transmission path. On the other hand, if the pumping light source 105 is ON (a solid line in the figure), the optical signal is amplified mainly at a latter half of the span due to an effect of stimulated Raman scattering by the pumping light supplied from the signal output end side of the transmission path, so that a part of the loss in one repeating span (span loss) is compensated. A gain of the Raman amplification at the time corresponds to a difference of the optical level for when the pumping light source is ON from the optical level for when the pumping light source is OFF, at the signal output end of the transmission path. Note, a chain line in the figure indicates a change in pumping light level.
As a control method of the distributed Raman amplifier in the system as illustrated in FIG. 8, there has been known an automatic level control (ALC) for controlling a supply state of the pumping light to the transmission path so that an optical output level per one channel in the WDM light which has been propagated through the transmission path to be Raman amplified, is fixed at a previously set target value according to an input dynamic range of the subsequent staged EDFA 103 (refer to Japanese Laid-open Patent Publication No. 2002-076482). Further, there has also been proposed an automatic gain control (AGC) for controlling the supply state of the pumping light to the transmission path so that a Raman gain in each span is fixed at a previously set target value (refer to Japanese Laid-open Patent Publication No. 2008-182679).
However, the distributed Raman amplifier applied with the above conventional control method has the following problems.
In the distributed Raman amplifier applied with the automatic level control (ALC), as illustrated in an upper stage of FIG. 10 for example, the target value of the optical output level is set in the vicinity of an upper limit of the input dynamic range of the WDM optical amplifier so that the OSNR (an average value of the OSNR of each channel) of the WDM light output from the EDFA or the like becomes maximum. Note, a lateral axis in the graph of FIG. 10 represents the span loss. However, if there is a difference between each span loss due to a difference in transmission path length of each span on the system (refer to A point and B point in the figure), the optical output level is fixedly controlled by the ALC of the distributed Raman amplifier, to the difference of the span loss, so that the Raman gain is changed (refer to G_A and G_B in the figure), and therefore, wavelength characteristics of the Raman gain are changed. If the wavelength characteristics of the Raman gain are changed, for example in the case where the wavelength characteristics of the Raman gain in each span are compensated for each span using an optical filter of which transmission wavelength characteristics are fixed (refer to a gain equalization filer (GEQ) 106 illustrated in FIG. 8), a compensation error in this optical filter is increased so that an OSNR of a specific channel among a plurality of channels contained in the WDM light is decreased. Accordingly, since a wavelength worst value of the OSNR is decreased, there is a possibility that transmission characteristics of all channels in the WDM light are hard to be improved.
Further, in the ALC of the distributed Raman amplifier, since the pumping light control is performed by detecting the power per one channel in the WDM light after Raman amplification, information relating to channel numbers of the WDM light is needed. Normally, this channel numbers information is acquired by utilizing an optical supervisory channel (OSC) transferred between nodes. However, since it takes a relatively long time to acquire the channel numbers information, there is a problem in that the ALC at a high-speed is hard to be performed. Further, if the target value of the optical output level is set in the vicinity of the upper limit of the input dynamic range of the EDFA or the like for performing the ALC of the distributed Raman amplifier, since the large pumping light power is needed, the power consumption in the distributed Raman amplifier becomes problematically large.
On the other hand, in the distributed Raman amplifier applied with the automatic gain control (AGC), even if there is the difference of the span loss on the system, since the Raman gain is fixedly held (refer to a lower stage of FIG. 10), the wavelength characteristics of the Raman gain are not practically changed. Therefore, it is possible to effectively suppress inter-wavelength deviation of the OSNR, which is problematic in the ALC. However, in the AGC, since the Raman gain of each span is controlled at the same target value irrespectively of largeness or smallness of span loss, the Raman gain which is same as that of the span in which the OSNR is decreased due to the large span loss, occurs in the span in which the excellent OSNR can be originally obtained since the span loss is small, and consequently, an inefficient control is performed. Namely, the Raman gain effective for improving the received OSNR in the repeating span of large span loss is set for the repeating span of small span loss, and therefore, it is problematically hard to efficiently improve the received OSNR.